In known recording apparatus, continuous recording of measurements is achieved by the marking of a record carrier by a recording arm deflected by the action of measuring impulses or signals.
The marking may be done by a pen mounted on the arm. Alternatively, a heating element may replace the pen and be used in combination with heat-responsive paper which undergoes a color change where adjacent to the element. A known third alternative involves the transference of a dry pigment tracing from a pigment or color carrier, such as a strip of carbon paper, to a strip-form record carrier under the pressure of the arm.
Such recording mechanisms are used in technical and scientific and particularly in biological and medical diagnostic recording apparatus such as electrocardiographs and electroencephalographs in order to record permanently and in directly visible form the changes of physical values, converted where desired into amplifiable current or voltage fluctuations, occuring in the subject under examination (the patient).
Such recording apparatus generally includes a means for converting electric signals comprising voltage or current fluctuations into a visible deflection of the recording arm. In operation of the above recording apparatus, the record carrier of, for example, paper, plastic or metal foil, is traversed through the recorder by means, usually in the form of rollers, for holding, guiding, tensioning and retarding the record carrier. A base, which is slightly yieldable or unyieldable is disposed in the recorder and is preferably in the form of an edge extending perpendicularly both to the direction of movement of the record carrier and also to the recording arm in its position of rest. The record carrier is passed between the base and the recording arm in directions which are inclined to one another, preferably at an obtuse or acute angle. The recording arm contacts the record carrier at the recording edge to produce a recorded trace.
The end of the recording arm remote from the record carrier is mounted for pivoting rotation so that the pivoting of the arm causes its marking end to travel across the face of the record carrier in a direction generally transverse to the direction of carrier movement.
This invention is more particularly related to the arm-deflecting drive system of such recorders, and is particularly suitable for rapid recording with single- and multi-channel lever arm strip churt recorders having a limited angle of rotation. The drive system typically includes a drive coil responsive to a deflection signal to deflect the arm by genrerating an electromagnetically induced pivoting force on the arm. The deflection signal is, in turn, generated by a differential amplifier responsive to the difference between two electrical signals. The first electrical signal is indicative of the angular position of the arm corresponding to the value to be recorded. The second electrical signal is indicative of the angular position of the arm and is produced by an angular position detector, subsequently described in greater detail.
The power amplifier amplifies the difference of the signals applied to its two input terminals, produces a correction signal corresponding to the difference between the desired nominal and actual values and applies this to the driver coil(s) of the drive system. The current flowing in the driver coil(s) effects a rotation of movement corresponding to the deviation of the angular position of the rotor from the nominal value. The suitable dimensioning of the power amplifier ensures the complete equilization of the control deviation.
Various types of angular position detectors for drive systems of recording elements have been used, employing a number of different principles: inductive angular position detectors operating according to the differential transormer principle, capacitive angular position detectors operating according to the differential capacitor principle, optical angular position detectors operating according to the principle of the differential pick-up with variable aperture openings for the light passing through the aperture.
All of the foregoing position detectors have required special measures for producing their functional readiness in that they have had to be supplied with suitably adapted exciter signals in accordance with their physical principles. Inductive and capacitive angular position detectors have required high frequency input voltages with possibly high voltage values. The required voltages necessitated costly electronic circuits, such as high frequency generators. In some cases, costly screening measures against magnetic and/or electric fields were necessary.
On the output side, inductive or capacitive angular position detectors additionally require electronic parts of the demodulation of the carrier-containing detector output signal. Demodulators, however, influence the phase angle of the demodulated signal in an undesirable manner. Since stable control of the closed control circuit, while keeping within the phase margin relationship, can be achieved only by reducing the upper cut-off frequency according to the changeover of the phase from a nonmodulated to a modulated angular position signal, demodulation results in a reduction in the upper cut-off frequency of the drive system.
Optical position detectors require light sources of high time stability and long duration. In addition, special measures are required against the ingress of extraneous light and/or undesirable reflections from the light source onto the light receiver.
All the foregoing angle position detectors have an additional common problem in that their use requires the addition of relatively massive elements to the pivoting arm, the polar moment of inertia of which mass, with respect to the polar moments of inertia of the remaining elements to be moved, is not insignificantly small. Because of the masses which have to be additionally driven, the total polar mass moment of inertia is increased in an undesirable manner, causing the upper cut-off frequency of the drive system to be ever further reduced. Since the movement of the additional mass requires additional torque, and the total available torque produced in the drive system is fixed, the ability of the arm to be quickly deflected is diminished.
It is therefore the aim of the invention to provide an angular position detector which eliminates the foregoing deficiencies respectively associated with commonly used angular position detectors.